Bipolar transistors are devices with two p-n junctions that are in close proximity to each other. A typical bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, the two p-n junctions, i.e., the emitter-base and collector-base junctions, are in a single layer of semiconductor material separated by a specific distance. Modulation of the current flow in one p-n junction by changing the bias of the nearby junction is called “bipolar-transistor action.”
If the emitter and collector are doped n-type and the base is doped p-type, the device is an “npn” transistor. Alternatively, if the opposite doping configuration is used, the device is a “pnp” transistor. Because the mobility of minority carriers, i.e., electrons, in the base region of npn transistors is higher than that of holes in the base of pnp transistors, higher-frequency operation and higher-speed performances can be obtained with npn transistor devices. Therefore, npn transistors comprise the majority of bipolar transistors used to build integrated circuits.
As the vertical dimensions of the bipolar transistor are scaled more and more, serious device operational limitations have been encountered. One actively studied approach to overcome these limitations is to build transistors with emitter materials whose band gaps are larger than the band gaps of the material used in the base. Such structures are called heterojunction transistors.
Heterostructures comprising heterojunctions can be used for both majority carrier and minority carrier devices. Among majority carrier devices, heterojunction bipolar transistors (HBTs) in which the emitter is formed of silicon (Si) and the base of a silicon-germanium (SiGe) alloy have recently been developed. The SiGe alloy (often expressed simply as silicon-germanium) is narrower in band gap than silicon.
The maximum oscillation frequency (fmax) of a HBT is limited by the device parasitics. More specifically, the lateral out-diffusion of collector dopant implant increases the perimeter parasitic component of the collector-to-base capacitance (CCB) near the trench isolation edge. This increase in CCB limits fmax of the HBT.
One approach to suppress the lateral diffusion of phosphorus and other like dopants is to incorporate carbon, C into the collector region. Specifically, C has been shown to suppress boron in the SiGe extrinsic base (See, for example, H. J. Osten, et al., “Carbon doped SiGe Heterojunction Bipolar Transistor for High Frequency Applications”, IEEE/BCTM, 1999, p. 169). Similarly, carbon can be incorporated in the collector to suppress the lateral diffusion of phosphorus.
U.S. Pat. No. 6,534,371 B1 to Coolbaugh, et al. describes a method in which carbon is incorporated into various regions (or parts) of a SiGe bipolar device to control or prevent bipolar shorts between the emitter, base and collector.
Despite the advancements made in the prior art mentioned above, there is still a need for providing a bipolar structure wherein the lateral out-diffusion of dopants, such as n-type dopants or p-type dopants, from the intrinsic device towards its perimeter is suppressed. The suppression of such dopant diffusion is critical for reducing the perimeter parasitic component of CCB.